Hydro-mechanical differential steering systems are employed in large track-type tractors or machines to provide smooth, equal, and uninterrupted power and torque flow to both the left and the right tracks at all times when the tractor is moving. It is also important to provide power to the tracks whenever the tractor is turning in either the left or right directions. A hydro-mechanical differential steering system is typically powered by a mechanical input and a hydraulic input. The mechanical input is provided from the transmission and the hydraulic input is received from an engine driven variable displacement pump. Other types of differential steering systems are also employed in track-type tractors and similar machines where steering is executed by differential motion of steering elements such as tracks and wheels.
In conventional machines, such as a track-laying type of machine, turning control is performed by an operator steering lever or other type of control mechanism. Even when under the control of the operator, the track-type tractor can have instances where uncommanded steering or other motion can occur. For example, the track-type tractor may move forward in a straight line when the operator tries to turn the machine. In other situations, the track-type tractor may turn where no turn is commanded by the operator, or may turn in the opposite direction that that commanded by the operator. The latter turn anomalies may be particularly problematic due to the risks of hitting workers or structures near the track-type tractor, or driving the track-type tractor over surfaces or terrain that may cause instability of the machine. Moreover, the uncommanded motion may be more problematic when the machine is operating at higher speeds where the operator has less time to react to and correct the uncommanded motion.
Control systems for hydrostatically driven equipment are known in the art. For example, U.S. Pat. Appl. Publ. No.2009/0133951, entitled “Method for Operating a Hydrostatically Driven Vehicle,” teaches a hydrostatically driven vehicle, such as an asphalt compactor, has an engine operating a variable displacement propel pump, a displacement of which can vary based on an angle of a rotating swashplate, such that a fluid flow impelled by the pump transfers power to at least one propel motor rotating a wheel of the vehicle. An electronic controller of the vehicle senses an operating parameter of the system, for example, the angle of the rotating swashplate or the direction and speed of rotation of the propel motor with a sensor to yield an actual signal, and relays the actual signal to an electronic controller. The controller determines a desired angle for the rotating swashplate based on the control signal, and compares it to the actual signal from the sensor. Motion of the vehicle is stalled when the angle signal differs from the desired angle by a predetermined extent and for a predetermined period. The control disclosed in the publication relates only to the actual speed of the vehicle and not to uncommanded turning motion as can be found in track-type tractors. Additionally, the predetermine period of time for evaluating the difference between the actual angle of the swashplate is different from the commanded angle appears to be the same regardless of the commanded speed of the vehicle.
In another example, U.S. Pat. Appl. Publ. No. 2008/0093152, entitled “Hydraulic Hybrid Vehicle Method of Safe Operation, teaches a hydraulic hybrid vehicle includes elements such as a hydraulic pump driven by an internal combustion engine and arranged to draw in low pressure fluid and pump the fluid at high pressure to an accumulator. A hydraulic motor is powered by the pressurized fluid. Safety processes are provided for detecting and addressing a number of conditions that may arise in the operation of the hydraulic hybrid vehicle, including an initialization procedure for start-up of the vehicle, a shut-down procedure, and procedures for detecting and responding to failure of the pump or motor, internal and external fluid leaks, and non-responsive actuation and mode control systems. One process monitors the motor displacement and compares a commanded motor displacement to an actual motor displacement. If the difference between the displacements is above a predetermined threshold value, the commanded motor displacement is set to zero. If the difference is still greater than the threshold value, it is determined that a condition similar to a throttle sticking is occurring and a mode valve is commanded to a neutral position and an auto-shutdown may be performed. The duration of the difference between the commanded and the actual motor displacements is not evaluated in the process, and the auto-shutdown may occur on the first instance of detecting the difference in the motor displacements.
In some prior implementations where a steering motor is used, for example, to cause a track-type tractor to turn by creating differential speed between the tracks, steering motor speed feedback is used to detect uncommanded motion occurring when the actual steering motor speed is above a commanded steering motor speed by a threshold speed for a specified amount of time. Once the threshold speed is exceeded for the prescribed time period, the brakes are engaged to stop the tractor. Using a fixed speed threshold and specified duration of uncommanded motion for all speeds of the tractor creates problems in tuning the uncommanded motion limiting system. If the speed threshold is lowered and/or the duration limit is shortened to react quickly to uncommanded motion at high tractor speeds, the number of false trips, and corresponding work delays, are increased. If the speed threshold is raised and/or the duration is lengthened, the false trips at low speeds will be reduced, but the delay in stopping the tractor for uncommanded motion at high speeds reduces the amount of time and distance available to stop the tractor and avoid an accident. If the thresholds are set to strike a balance between high speed and low speed operations, the system is not optimized for either condition and will still result in some false trips at very low speeds and some excess uncommanded motion at very high speeds.
The type of system described above also fails to account for operating conditions internal and external to the vehicle that affect the vehicle's ability to respond to uncommanded motion. For example, the temperature of oil affects the oil's viscosity. The oil is more viscous at lower temperatures, and the vehicle systems are generally less responsive or sluggish when the oil temperature is low. As the temperature increases, such as when the running engine is transferring heat to the lubricating oil, the viscosity of the oil decreases and the vehicle systems become more responsive to operator commands and react more quickly to the commands to steer the machine. By using a speed threshold and a specified duration for determining when excess uncommanded motion is occurring, an uncommanded motion limiting system wholly ignores any affects of the internal and external conditions affecting vehicle response.
In view of the above, a need exists for a system for detecting uncommanded motion of a steering motor and alerting an operator of the vehicle that may take into account and base its responsiveness on the steering motor speed and the cumulative effect of the uncommanded motion over a number of samplings, the effect of internal and external factors affecting the speed of reaction of the vehicle to correct or eliminate uncommanded motion, as well as other factors.